1. Technical Field
The present invention relates to a piezoelectric vibrator of a thickness vibration mode, and more particularly, to a piezoelectric vibration element, a piezoelectric vibrator, a piezoelectric oscillator, and an electronic device having so-called a mesa structure.
2. Related Art
AT cut quartz crystal vibrators have a thickness-shear vibration as its vibration mode and are appropriate for miniaturization and the implementation of a high frequency, and the frequency-temperature characteristic thereof represents a superior cubic curve. Accordingly, the AT cut quartz crystal vibrators are widely used for electronic apparatuses and a variety of other uses.
In JP-B-58-045205, so-called a piezoelectric vibrator having a mesa structure, which has an energy trapping effect similarly to a beveled structure or a convex structure, is disclosed, and a piezoelectric substrate having a circular shape and a circular mesa structure is disclosed.
In JP-A-58-047316, in addition to a piezoelectric substrate having a circular shape and a circular mesa structure, a piezoelectric substrate having a strip-shaped mesa structure is disclosed.
As the sizes of the electronic apparatuses tend to decrease, a vibrator having a low side ratio (the ratio of a longer side or a shorter side to the thickness) has been requested. In a vibrator having a low side ratio, a contour vibration may be easily combined with its main vibration, and accordingly, the electrical characteristics of the main vibration deteriorate.
In JP-UM-A-06-052230, an AT cut quartz crystal vibrator is formed so as to employ a mesa structure, and a problem is handled in which a drawn-out electrode (lead electrode) extending from an excitation electrode may be broken in case where the side wall of a boundary portion between a mesa portion and a thin-walled portion forms 90° with respect to the principal face, and it is disclosed that the breaking of the lead electrode can be prevented by forming the side wall of the boundary portion to be tilted or a curved face. In addition, it is disclosed that, by decreasing the roughness of the surface of the vibration portion to average roughness of 0.2 micron, the CI value decreases, and secondary vibration is suppressed.
In addition, in JP-A-2001-230655, a quartz crystal vibrator is disclosed in which an AT cut quartz crystal vibrator is formed so as to employ a mesa structure, and the side walls of a mesa portion are tilted to 63° and 35°, thereby suppressing a combination of thickness-shear vibration and bending vibration.
In Japanese Patent No. 4,341,583, it is disclosed that, when the frequency of a quartz crystal vibration element is f, the length of the longer side (X axis) of a quartz crystal substrate is X, the thickness of a mesa portion (vibration portion) is t, the length of the longer side of the mesa portion is Mx, the length of the longer side of an excitation electrode is Ex, and the wavelength of bending vibration occurring in the longer side direction of the quartz crystal substrate is λ, by setting the parameters f, X, Mx, and Ex so as to satisfy the following four equations, a combination of thickness-shear vibration and bending vibration can be suppressed.λ/2=(1.332/f)−0.0024  (1)(Mx−Ex)/2=λ/2  (2)Mx/2=(n/2+¼)λ (here, n is an integer)  (3)X≧20t  (4)
In addition, in Japanese Patent No. 4,341,671, it is disclosed that a bending displacement component decreases in a case where the positions of the end edge of a vibration portion and the end edge portion of an excitation electrode are set so as to coincide with the position of the antinode of the bending displacement, whereby the bending vibration as an unnecessary mode can be suppressed.
In JP-A-2008-306594, a mesa-type vibration element that improves the frequency variable sensitivity and suppresses unnecessary vibration is proposed. Generally, in a vibration element, as an excitation electrode is formed to be larger, the equivalent series capacitance C1 increases, whereby the frequency variable sensitivity can be increased. It is disclosed that a mesa-type vibration element in which the excitation electrode is formed to be large can be easily oscillated, and the width of the frequency change with respect to load capacitance can be broadened.
In JP-A-2009-065270, it is disclosed that, when the longer side of a piezoelectric substrate is in the X axis direction, the shorter side thereof is in the Z′ axis direction, the longer side of a mesa portion is in the X axis direction, and the shorter side (length MZ) thereof is in the Z′ axis direction in a mesa-type vibration element, both end portions of one shorter side of the mesa portion are chamfered, and the length thereof is M1, by satisfying the relationship of M1≦Mz/4, bending vibration can be suppressed.
In JP-A-2009-130543, an AT cut quartz crystal vibrator having a mesa structure is disclosed. When the positions of both end portions of a mesa portion along the X axis direction are A and D, and the positions of excitation electrodes formed on the mesa portion are B and C, the relationship of A<B<C<D is configured to be satisfied. The positions of the end edges A, B, C, and D and the position of the antinode of bending vibration are configured to coincide with each other. The amplitude of the antinode of bending vibration that is located at the position A and the amplitude of the antinode of bending vibration that is located at the position B are in opposite directions. In addition, the amplitude of the antinode of bending vibration that is located at the position B and the amplitude of the antinode of bending vibration that is located at the position C are in opposite directions. Furthermore, the amplitude of the antinode of bending vibration that is located at the position C and the amplitude of the antinode of bending vibration that is located at the position D are in opposite directions. In other words, when the wavelength of the bending vibration is λ, in the end edges adjacent to each other, there is misalignment of the antinode of the bending vibration of odd times λ/2. It is disclosed that, when the length of the mesa portion is ML and the wavelength of the bending vibration occurring in the X axis direction is λ, by configuring ML and λ so as to satisfy the relationship of ML=(n−½)λ, the bending vibration occurring in the mesa-type vibration element can be suppressed, and the CI can be decreased.
There is a problem in that the capacitance ratio γ (the ratio C0/C1 of electrostatic capacitance C0 to the equivalent serial capacitance C1) of the piezoelectric vibration element employing the mesa structure is relatively high (deteriorates), compared to those of the piezoelectric vibration element employing the beveled structure or the convex structure. In Japanese Patent No. 4,506,135, a piezoelectric vibration element is disclosed in which an excitation electrode is broadened toward the end face of the piezoelectric substrate more than the level difference portion of the mesa portion. By changing the area of the excitation electrode of the portion enlarged toward the outer side more than the level difference portion of the mesa portion, the capacitance ratio of the piezoelectric vibration element can be arbitrarily set. As a result, it is disclosed that a piezoelectric vibration element employing a mesa structure that has performance equivalent to a piezoelectric vibration element employing the beveled structure or the convex structure can be realized.
In Japanese Patent No. 4,558,433, a piezoelectric vibration element is disclosed which suppresses variations of the oscillation frequency due to a change in the load capacitance CL by increasing the capacitance ratio γ of the fundamental wave vibration. Generally, the capacitance ratio γ is configured to be low by configuring the area of the excitation electrode near a saturation point of the equivalent serial capacitance C1. However, by further broadening the area of the excitation electrode passing the saturation point, while a change in the equivalent serial capacitance C1 is small, the electrostatic capacitance C0 increases in proportional to the area. Accordingly, it is disclosed that the capacitance ratio γ can be increased.
In JP-A-2001-230654, a piezoelectric vibrator is disclosed in which excitation electrodes are disposed on the front and rear faces of a narrow band-shaped (strip-shaped) piezoelectric substrate at the center portion, and lead electrodes extend from the excitation electrodes toward end portions located on the opposite sides. The substrate of the face on which the lead electrode is not formed is ground so as to form a pseudo mesa-type structure. Since this piezoelectric vibrator can trap the vibration energy under the excitation electrode, it is disclosed that the CI is low, and it is difficult for breaking of the lead electrode to occur.
In JP-A-2010-062723, it is disclosed that, when the length of the shorter side of a piezoelectric substrate employing a mesa structure is Z, the thickness of a mesa portion (vibration portion) is t, and the dimension of the electrode in the shorter side direction of the mesa portion is Mz, by setting parameters so as to satisfy the relationship of 15.68≦Z/t≦15.84 and 0.77≦Mz/Z≦0.82, an unnecessary mode can be suppressed.
In Japanese Patent No. 4,572,807, a mesa-type piezoelectric vibration element is disclosed. It is disclosed that, when the length of the longer side of a quartz crystal substrate is x, the canal amount (the height of a mesa portion) of the level difference portion is Md, the plate thickness of the vibration portion is t, and the ratio of the canal amount Md of the level difference portion to the plate thickness t is y (percentage), by configuring the ratio y to satisfy the relationship of y=−1.32×(x/t)+42.87 and y≦30 and the ratio of the length x of the longer side to the plate thickness of the vibration portion of the quartz crystal substrate, that is, the side ratio x/t to be equal to or less than 30, the electrical characteristics of the piezoelectric vibration element do not deteriorate, and the CI can be decreased.
In JP-A-2008-263387, it is disclosed that, when the dimension of the longer side of a piezoelectric substrate is x, the thickness dimension of a mesa portion (vibration portion) is t, and the height (canal amount of the level difference portion) of the mesa portion of the piezoelectric substrate employing a mesa structure is y, by setting the side ratio x/t so as to satisfy “y=−0.89×(x/t)+34±3(%)” with reference to the plate thickness t, an unnecessary mode can be suppressed.
However, recently, the miniaturization of a piezoelectric vibrator so as to configure the container size to be about 1.6 mm×1.2 mm is requested from customers. The X side ratio (the ratio X/t of the longer side X to the thickness t) of an AT cut quartz crystal vibrator element employing a mesa structure that is mounted in such a small container, for example, is equal to or less than 1100 (μm)/65 (μm)=17. Even by applying the above-described techniques to such as small-size piezoelectric vibrator, there is a problem in that the CI (crystal impedance, equivalent resistance R1) requested from the customers cannot be acquired.